A fundamental problem in a cable termination is to manage the transition from cable insulation to air as an insulating medium without electrical flashover. The insulating resistance of air is low in relation to the cable insulation. Therefore, it is necessary, in a cable termination, to control the electric field such that the field strength in the air drops to a suitable level. In a cable with a solid insulation, a cable termination is often designed with a field-controlling insulating body field-controlling member which has a relative permittivity of the same order of magnitude as that of the cable insulation. The insulating body has an outer conducting layer which is connected to the outer conducting layer of the cable. Such an insulating body may be regarded as a thickening of the cable insulation. By forming the body in the correct way, the field strength will be sufficiently small on the outside of the body, so that the insulating resistance of the air is not exceeded. A cable termination is also influenced by the external environment and must be designed to cope with pollution. In case of outdoor erection, the termination also has to be designed to withstand rain and salt precipitation. As a consequence of pollution, leakage currents arise along the exposed insulator surfaces.
To obtain a voltage-grading termination in a high-voltage cable with an insulation of crosslinked polyethylene (PEX), a field-controlling member, a so-called stress cone, of insulating elastic material, partially surrounded by a conducting external coating, is previously known. The stress cone is intended to control the electric field strength around the cable end such that the field-strength gradient along the cable insulation is kept sufficiently low to avoid partial discharges and hence a risk of breakdown in the insulating medium surrounding the cable termination. At low voltages, the insulating medium normally consists of air, which provides simple, inexpensive terminations which are easy to handle. At high voltage, however, such devices become bulky. Cable terminations of this type intended for high voltage are complicated to assemble. Nor do they permit the components to be tested in assembled state prior to energization. The possibility of an assembly with products tested in advance is, however, desirable in the industry of today and of great economic importance.
From EP 0 017 953, a cable termination for a cable with a solid insulation of the above-mentioned kind for a medium-voltage network is previously known. The cable conductor is here surrounded by an insulating body of silicone rubber, in which a deflector is embedded which forms an orifice for the outer conducting layer of the cable. For high system voltages, however, this cable termination, in order not to exceed the insulating resistance at the insulating body, must be designed with a very long and thick insulating body. This makes the cable termination unstable and bulky. It is mounted directly on the stripped cable conductor, which requires extensive work at the place of installation.
In order to obtain reasonable dimensions of a cable termination at high or medium voltage, a PEX cable is often provided with an oil-insulated termination in an electrically insulating enclosure of plastic or ceramic material. Such a design, however, also entails increased costs of the insulating oil and its insulating enclosure, as well as costs of expansion space for the thermal expansion of the insulating oil. A further problem with the terminations filled with insulating oil is the risk of leakage, which constitutes an environmental problem. The risk of leakage also entails a potential risk of flashover.